Hole transport in molecularly doped polymers (MDPs) is modeled as random walks on fixed donors (Ds) embedded in a polymer matrix. Dilution p<1 corresponds to placing individual Ds, dimers D2, or tetramers D4 randomly on a fraction p of sites in a face-centered-cubic lattice. Monte Carlo simulations of the drift velocity vD(E) in a bias field E have maxima in dilute (p = 8%) systems of D2 or D4 that are related to the formation and polarization of clusters of nearest-neighbor donors. Marcus or...